The present invention relates to the cutting teeth on a metal-cutting saw.
Saws for cutting metal are commonly made with a base of flexible steel of uniform thickness, and teeth projecting from an edge of the base. Some of the teeth are set to produce a cut which is wider than the thickness of the base, to avoid friction between the base and the sides of the cut. Each tooth is usually provided with a tip formed of a material which is harder and more wear resistant than the base, such as high speed steel or cemented carbide. The cutting edges on the tooth tips are commonly sharpened by grinding, to minimize the cutting force. The saws can be made as continuous loops or as band saws, or straight strips such as hack saws.
For the shape of the teeth, numerous variations have been suggested, as has been shown in U.S. Pat. Nos. 2,637,355, 4,727,788, 4,179,967 and others. The great variation of tooth shapes is the result of the many requirements on the saw performance, such as ability to cut different metals, to cut with reduced vibrations, to produce smooth cut surfaces, to be produced with low cost and high precision, and to remove metal chips from the cut. For optimal performance in difficult applications it may be necessary to use specific tooth shapes which may be clearly different from normal tooth shapes. Similar requirements also apply to saws for cutting wood, as described in U.S. Pat. No. 4,590,837 and others, although the fibrous character of wood will require completely different tooth shapes that are not applicable to metal saws.
Normally, the ability to make a straight cut in metal is achieved by providing the set teeth with sloping cutting edges (e.g., see the sloping edges 32a of FIG. 3), so that a deviation of the saw to one side (e.g., away from the center of the cut) will increase the chip thickness and cutting force of the teeth set to that side, returning the saw to the previous center of the cut. When entering a cut, a straight cut is achieved by including, along with set teeth, teeth which are straight and symmetrical to a center plane of the base (i.e., non-set), taller than the set teeth and preferably provided with chamfers to make the lateral force component linearly dependent on the deviation from a straight line of cut.
For long cuts through thick metal, the vibrations can be minimized by varying the distances between teeth as is well known. However, when a great number of teeth are cutting at the same time, the force required to convey the chips becomes important.
When cutting metals, the cutting force depends to a large extent upon the friction force to convey the chips out of the cut. This is most noticeable for soft pliable metals as aluminum, where the chips curl up into large tight rolls before they fracture. If such rolls rub against the walls of the cut, they will cause friction, scar the walls and block further chip formation, especially if they are twisted or turned around. In the sawing of regular carbon steel, the chips fracture in small pieces before they curl up. This means that common saw tooth shapes for carbon steel (such as in FIG. 3) will not be suitable when sawing aluminum.
One common feature of metal-cutting saw teeth is that the cutting edges of set teeth (i.e., the cutting edges 32a of the teeth 32 in FIG. 3) form blunt obtuse angles .alpha. with the walls (31) of the cut, because they become inclined during the setting process and possibly chamfered as disclosed in U.S. Pat. No. 2,635,327. Straight teeth (33 or 41) also form blunt angles with the walls of the cut, because they are made with chamfers, as described in U.S. Pat. No. 5,331,876. This means that short fractured chips will be deflected towards the center of the cut, minimizing the friction and scarring of the walls. Curled chips, however, will be shaped as rolls with a roll-up axis parallel to the cutting edge and will rub and chafe against the walls (31) as soon as they encompass a full turn.
Inclining the tops of the teeth the other way to form an acute angle rather than an obtuse angle with the walls is common for saws that cut fibrous materials like wood as shown in FIG. 5 of U.S. Pat. No. 4,590,837. Such teeth will also deflect chips towards the center, because the cutting edge in that case is not along the top of the tooth, but rather along the side of the tooth.
In saws for sawing metals, as in FIGS. 3 and 4, the tooth top is the cutting edge, and if the angle .alpha. were acute, the chips would rub against the wall (31) right from the beginning even if they were soon fractured. Providing the set teeth with edges 42a at a right or nearly right angle has been suggested in FIGS. 4a and 4b of U.S. Pat. No. 4,727,788, and in DE G8807350.5 (shown herein in FIG. 4), but in both cases this requires a grinding operation after setting, which may be difficult and expensive since it is performed without proper back support and on selected teeth only.
The stable cutting of a straight cut requires that lateral deviations of the saw produce well-defined lateral force components proportional to the deviations. In most metal cutting saw designs, those force components are produced by the inclined edges of the set teeth, but if the edges were instead at right angles to the side wall (31), the lateral forces could be produced by large symmetric chamfers (43) formed on the straight teeth (41), as shown in FIG. 4. Chamfered straight teeth have the additional advantage of producing chips of restricted width, which are easily conveyed, but also have the severe disadvantage of requiring three separate grinding operations with very strict symmetry. Also, the corner (45) where the chamfer meets the top edge is also subject to more wear than straight edge portions.